The present invention relates to a composite longitudinal vibration mechanical filter which comprises longitudinally vibratable bodies (hereinafter also referred to as "longitudinally vibratable tuning bars"), piezoelectric elements, coupling elements, and supporting elements, and which is capable of appropriately reducing frequency fluctuations due to different lengths of the longitudinally vibratable tuning bars, undesired spurious responses, and passband deteriorations, or of adjusting the resonant frequency, when desired frequency characteristics are created by the transmission of composite longitudinal vibration, and a method of manufacturing such a composite longitudinal vibration mechanical filter.
Recently, mechanical filters having characteristics which are of an intermediate level as compared with those of LC filters and quartz filters are widely used in communication devices. Such mechanical filters have a good Q factor, a good selectivity, and a good temperature characteristic, and can be reduced in size.
One conventional composite longitudinal vibration mechanical filter is shown in FIG. 1 of the accompanying drawings. The mechanical filter has an input longitudinally vibratable tuning bar 2 and an output longitudinally vibratable tuning bar 4 which are disposed in the same plane and are made of a metal material. Identity elastic coupling elements 6, 8 are joined to the input and output longitudinally vibratable tuning bars 2, 4, and supporting elements 10, 12 project outwardly from the centers of the tuning bars 2, 4. The tuning bars 2, the coupling elements 6, 8, and the supporting elements 10, 12 are fabricated by precision pressing and joined together by laser welding or the like. A pair of input piezoelectric ceramic members 14a, 14b is superposed on and fixed to the input longitudinally vibratable tuning bar 2 by soldering or the like, and similarly a pair of output piezoelectric ceramic members 16a, 16b is superposed on and fixed to the output longitudinally vibratable tuning bar 4 by soldering or the like. The supporting members 10, 12 have outer ends secured to upper central surfaces of upstanding members 24a, 24b, respectively, of a holder 24 by laser welding or the like.
A feed line 18 and a grounding line 18e, between which an input signal is supplied, are connected to the input piezoelectric ceramic members 14a, 14b and the upstanding member 24a, respectively. Likewise, an outlet line 20 and a grounding line 20e, from which an output signal is led out, are connected to the output piezoelectric ceramic members 16a, 16b and the upstanding member 24b, respectively.
The input and output longitudinally vibratable tuning bars 2, 4, which are coupled to each other by the coupling elements 6, 8, are held in space so that they can be longitudinally vibrated unobstructedly. The composite longitudinal vibration mechanical filter is housed in a casing (not shown), which is mounted in an intermediate frequency amplifier in a communication device or the like.
The composite longitudinal vibration mechanical filter shown in FIG. 1 operates as follows: A high-frequency signal S.sub.1 produced by a signal source Osc is supplied to a resistor R and then fed to feed line 18 and the grounding line 18e, and applied to electrodes (not shown) attached to the input piezoelectric ceramic members 14a, 14b. The applied high-frequency signal S.sub.1 generates an electric field having the same frequency as that of the signal S.sub.1, between the electrodes and the input longitudinally vibratable tuning bar 2 which is electrically grounded. In response to the electric field thus generated, the input piezoelectric ceramic members 14a, 14b are mechanically deformed in the directions indicated by the arrows Vm, Vn in FIG. 1, and the input longitudinally vibratable tuning bar 2 resonates to produce a longitudinal wave having a frequency F.sub.1 and a half wavelength which is equal to the length L.sub.1 of the input longitudinally vibratable tuning bar 2. If the longitudinal wave propagates along the input longitudinally vibratable tuning bar 2 at an average speed V, then the frequency F.sub. 1 is given by the following equation: EQU F.sub.1 =V/(2L.sub.1) (1)
The longitudinal vibration of the input longitudinally vibratable tuning bar 2 is mechanically coupled and propagated to the output longitudinally vibratable tuning bar 4 to the coupling elements 6, 8, causing the output longitudinally vibratable tuning bar 4 to resonate or vibrate longitudinally at a frequency F.sub.2 and with a half wavelength equal to the length L.sub.2 of the tuning bar 4. If the longitudinal wave is propagated to the output longitudinally vibratable tuning bar 4 at an average speed V, then the frequency F.sub.2 is given by the following equation: EQU F.sub.2 =V/(2L.sub.2) (2)
The longitudinal vibration of the output longitudinally vibratable tuning bar 4 produces a voltage between the output piezoelectric ceramic members 16a, 16b. The produced voltage is then led out between the outlet line 20 and the grounding line 20e as a high-frequency signal S2 having a sharp frequency characteristic curve.
In the process of manufacturing the composite longitudinal vibration mechanical filter shown in FIG. 1, much importance is attached to the accuracy of a central frequency and the bandpass characteristics of the produced mechanical filter, and it is desired that the resonant frequencies F.sub.1, F.sub.2 of the input and output longitudinally vibratable tuning bars 2, 4 have the same central frequency. However, since the input and output longitudinally vibratable tuning bars 2, 4 are mass-produced in large quantities by etching or precision pressing, it is difficult to give the individual components a sufficient level of dimensional accuracy. As a result, the mass-produced mechanical filters have different central frequencies and relatively poor bandpass characteristics.
The feed line 18 and the outlet line 20 are spaced from each other to reduce the inductive coupling therebetween due to a stray capacitance, i.e., to increase the isolation therebetween. However, since an undesired vibratory wave which is produced by the input longitudinally vibratable tuning bar 2 is transmitted to the output longitudinally vibratable tuning bar 4 via the coupling elements 6, 8 and the supporting elements 10, 12, unwanted spurious responses are created outside of the passband of the mechanical filter.